1. Field of the Invention
The present invention relates generally to a bushing for a jet engine and more specifically to an improved bushing which is less expensive to manufacture and which facilitates shipping and installation of the bushing.
2. Discussion of the Related Art
Jet engines generally include an axial air compressor which supplies compressed air into a combustor. The front section of a jet engine includes the axial air compressor. The axial air compressor generally includes several consecutive stages, each having a number of stator (stationary) vanes in a shroud and an equal number of rotor (rotating) vanes. Rotor vanes are designed and arranged such that, as the rotor vanes pass by the stator vanes in a particular stage, they take in a volume of air, compress the air and pass this compressed air into the next stage for further compression of the air. Some jet engines, such as the CFM56-2 turbofan engine manufactured by General Electric for example, have thirteen stages of compression. Stages 1 through 5 of this engine have stator vanes in which the pitch of the vanes is variable. The pitch of the variable stator vanes can be adjusted to vary the volume of air intake and thereby control the volume and pressure of the air that is subsequently injected into the combustor to be combined with fuel and ignited. The thrust of the engines can thereby be varied and the amount of air can be metered accurately for maximum fuel consumption. This is desirable because it provides the pilot with greater control over the amount of thrust produced within the engine at given engine speeds. For example, when the pilot is bringing the jet in for a landing, he or she can keep the engine running at very high RPMs and vary the pitch of the vanes to generate less thrust within the engine. If there is a problem and the pilot has to quickly generate thrust to pull up, he or she simply rotates the vanes of the stator to a pitch which increases the amount of air directed into the stator, thus increasing the thrust produced by the engine. The adjustable vanes allow the pilot to quickly produce thrust without having to adjust the RPMs at which the rotor is rotating.
As shown in FIG. 7, each vane 10 of the engine described above includes a blade 12 which is rotatably mounted between an inner shroud 14 and an outer shroud 15. It will be understood that each stator of a jet engine may include several stages, each having many adjustable vanes. However, for simplicity, only one vane is shown in FIG. 7. Vane 10 includes a spindle, partially shown at 16 in FIG. 7 and fully shown at 16 in FIG. 1, which is held in place between portions 20a and 20b of inner shroud 14 within aperture 18. Vane 10 also includes a drive portion 22 mounted within outer shroud 15. A steering mechanism (not shown) is coupled to the drive portion 22 to rotate the vane 10 within the inner shroud 14 and the outer shroud 15.
In order to facilitate the rotation of the vane 10, a bushing is mounted on spindle 16 before it is mounted between portions 20a and 20b of inner shroud 14. A prior art bushing 24 is shown in FIGS. 1 and 2. Since the operating temperature of the jet engine can reach 550.degree. F., bushing 24 must be made from a material that can withstand the extremely high temperatures to which it will be subjected. Therefore, bushing 24 is typically formed from a plastic which is capable of withstanding these temperatures. One prior art bushing 24 is formed from a plastic material sold by DuPont under the trademark VESPEL. However, this material is not capable of being melt processed, meaning that it cannot be used in an injection molding process to form the bushing 24. Bushing 24 is typically formed from a billet of the VESPEL material and is machined to the shape shown in FIG. 1, including a circumferential groove 28 (FIG. 2). The bushing 24 is then cut in half to form parts 26a and 26b. An elastic band 30, made from a high temperature-resistant material, is placed within groove 28 to hold parts 26a and 26b together on spindle 16 until the spindle is mounted to inner shroud 14, as described above.
Due to the properties of the material used in the manufacture of bushing 24, the requirement that each bushing be separately machined and the requirement for the elastic band 30, bushing 24 is very time consuming and expensive to manufacture. Shipping the bushings from the manufacturer to the end user is problematic because the three-piece bushings are prone to disassembling during shipping, thus requiring extra time for reassembling the bushing before it is installed on the vane 10. Furthermore, since each part 26a and 26b must be held in place on spindle 16 while the elastic band 30 is installed, the installation of bushing 24 on the vane 10 is very time consuming. Since, every time a jet engine is rebuilt, every vane bushing is replaced, the replacement of the bushings adds considerably to the expense and time required to rebuild an engine.
What is needed is a vane bushing for a jet engine which is simple and inexpensive to manufacture, and which is easy to ship and install on a jet engine vane.